Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and a photo-detector being integrated within a pickup. The photo-detector is used for detecting the reflected light of the laser beam when reading data from the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store up to 50 GB on a dual layer disc. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2T to 8T and 9T is used, where T is the channel bit length, and which corresponds with a minimum mark length of 138-160 nm. The re-writable BD-RE disc is based on a phase change technology comprising a phase change layer, which uses for example a compound of AgInSbTe or GeSbTe. Further information about the Blu-Ray disc system is available for example from the Blu-Ray group via internet: www.blu-raydisc.com.
New optical storage media with a super resolution near-field structure (Super-RENS) offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-Ray disc. This is possible by a so-called Super-RENS structure, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The super resolution layer is a non-linear layer which is also called a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Materials applicable as a Super-RENS structure are for example phase-change materials from which it is known that they become transparent when they are heated by a laser. Applicable are also some semiconductor materials which show a strong increase in reflectivity for higher laser intensities, the reflectivity being dependent on the pit structure of a corresponding data layer of the storage medium.
U.S. Pat. No. 5,365,052 discloses an apparatus with an optical system for reading information on an optical storage medium, wherein the effective numerical aperture of the light receiving optical system is larger than the effective numerical aperture of the focusing optical system. The focusing optical system includes an objective lens and the light receiving optical system includes a light receiving means disposed around the objective lens for receiving reflected light not passing through the objective lens.
U.S. Pat. No. 6,920,091 describes an apparatus comprising a pickup having a first and a second light shade for separating reflected light of an optical disc into a center portion and a peripheral portion, wherein the light of the center portion is exclusively used for signal reproduction, so that the signal-to-noise ratio is increased. The optical disc is an aperture type Super-RENS optical disc.
U.S. Pat. No. 5,802,036 discloses an apparatus with a pickup utilizing a super-resolution technique with a super-resolution filter for weakening the light intensity of the center portion of light passing through the objective lens, which reduces the size of the light spot on the optical storage medium. The reflected light is collected by the objective lens with a larger numerical aperture with regard to the laser beam impinging on the optical storage medium.